This invention is directed to providing industrial coolants, characterized by high viscosity and a poor heat transfer coefficient, flowing through an evaporator at a rate of 200 grams/sec (2 gallons/main) at temperatures between −80 and −100 C. More particularly this invention defines a highly efficient and compact cylindrical evaporator construction intended to provide cooling to Galden TH 70 (a commercial coolant) while maintaining a pressure drop across the evaporator not exceeding 2 PSI. The thermal masses of the evaporator and of the coolant in contact with the evaporator during the initial cool down of the evaporator are minimized.
Refrigeration systems have been in existence since the early 1900s, when reliable sealed refrigeration systems were developed. Since that time, improvements in refrigeration technology have proven their utility in both residential and industrial settings. In particular, very low temperature refrigeration systems, colder than −20 C, currently provide essential industrial functions in biomedical applications, cryoelectronics, coating operations conducted in a vacuum (i.e. physical vapor deposition), semiconductor manufacturing applications, control of chemical reactions and pharmaceutical manufacturing processes. Another application involves thermal radiation shielding. In this application large panels are cooled to very low temperatures. These cooled panels intercept radiant heat from vacuum chamber surfaces and heaters. This can reduce the heat load on surfaces being cooled to lower temperatures than the panels. Yet another application is the removal of heat from objects being manufactured. In some cases the object is an aluminum disc for a computer hard drive, a silicon wafer for an integrated circuit, or the material for a flat panel display. In these cases the very low temperature provides a means for removing heat from these objects more rapidly than other means, even though the object's final temperature at the end of the process step may be higher than room temperature. Further, some applications involving hard disc drive media, silicon wafers, or flat panel display material, include the deposition of material onto these objects. In such cases heat is released from the object as a result of the deposition and this heat must be removed while maintaining the object within prescribed temperatures. Cooling a surface, like a platen, is the typical means of removing heat from such objects.
In many of these applications, such as the semiconductor device manufacturing industry, it is necessary that refrigeration systems provide very low temperature refrigeration to highly viscous industrial coolants with poor heat transfer coefficients. Highly viscous coolants provide several challenges and limitations to such systems, especially the evaporator of the refrigeration system. Additionally, in many such applications evaporator designs are further limited by size restrictions and a necessity to be compatible with customer systems already in place. A coolant is used as an intermediate fluid instead of direct thermal contact with the refrigerant in cases where the process tubing and heat exchanger are not rated for high design pressures required by refrigeration processes. Using a secondary coolant (usually a liquid, but sometimes a gas) allows the process at the load where the heat is removed to be operated with the coolant at much lower pressure than the pressure that the refrigerant process must operate.
This invention relates to refrigeration systems which provide refrigeration at temperatures between −20 C. and −150 C by use of a secondary cooling fluid or coolant. The temperatures encompassed in this range are variously referred to as low, ultra low and cryogenic. For purposes of this application the term “very low” or very low temperature will be used to mean the temperature range of −20 C. to −150 C.
Industrial applications that require very low temperature cooling often find it necessary to provide such cooling to liquid coolants that become highly viscous at such temperatures. As a liquid coolant is pumped through a closed loop system, the pressure drop experienced by the coolant as it flows through the evaporator affects the heat load on the refrigeration system, since higher coolant pressure drops require greater pump work. Greater pump work results in a greater increase in the fluid temperature rise during the pumping process and results in a higher heat load of the refrigeration system.
Many system configurations require limitations on the size of components, such as an evaporator. In the case of evaporator size restrictions, it is still necessary for the evaporator to supply the required cooling, or it is of no use. Evaporators typically achieve improved heat transfer effectiveness and overall refrigeration cycle efficiencies by including a larger heat transfer area. However, the inclusion of a large heat transfer surface area in a limited volume may present a significant challenge.
Many systems also require a quick initial cool down time. This is complicated by the high viscosity and low thermal conductivity of very low temperature industrial coolants. These physical limitations tend to result in larger heat exchangers since tight fin spacing increases pressure drop. Larger heat exchangers have more mass to be cooled on initial cool down. In addition, a larger heat exchanger typically requires a larger volume to be filled with liquid. This large volume of liquid represents an additional mass to be cooled initially. Therefore, an effective design minimizes heat exchanger mass and coolant volume, while maximizing flow passages (fin spacing). Due to the very high viscosity of the fluid, heat exchangers often will be operating with the fluid flow in the laminar flow regime. To minimize pressure drop, very low Reynolds number flow is required. A limitation of laminar flow is that fully developed laminar flow is difficult to alter in a way that enhances the heat transfer rate. Therefore, an effective, compact design must prevent fully developed flow. Understanding of fully developed flow and the development of fully developed flow relates to the formation of a boundary layer. The physics of boundary layers is well known to those skilled in the art of heat exchanger design. For reference, an excellent discussion of laminar flow heat transfer in boundary layers and fully developed laminar flow is given by “Convective Heat and Mass Transfer,” Kays and Crawford, McGraw Hill, 1980.
Galden TH 70 is an industrial coolant widely used in the semiconductor manufacturing industry characterized by high viscosity (especially at cold temperatures), a poor heat transfer coefficient, and a tendency to freeze out at temperatures below −120 C. These characteristics present many limitations and challenges to the design of an evaporator that is to remove heat from such a coolant.
Similar limitations are also experienced by other liquid heat transfer coolants used to provide heat transfer at temperatures below −20 C. Although the current design was originally developed for use with Galden TH 70 it can also be used for other similarly high viscous liquids.
Such very low temperatures are needed for a variety of industrial applications. In the semiconductor industry such very low temperatures are important for processing of semiconductor wafers. In one such example, the deposition of material on a wafer causes heat to be rejected to the wafer, which heat must be removed. Further, such processes must take place within a specified temperature range. Frequently, the process design requires cooling temperatures of −20 C. or colder to achieve desired process conditions. Additionally, very low temperature cooling is needed when the completed wafers are tested.